a) Field of the Invention
The invention is directed to a reflectometer arrangement for determining the reflectance of selected measurement locations on measurement objects reflecting in a spectrally dependent manner, with a polychromatically emitting radiation source whose radiation is directed onto the measurement object as a measurement beam bundle, and with a device for receiving reflected radiation from the selected measurement location. The invention is further directed to a method for determining the reflectance of selected measurement locations on measurement objects reflecting in a spectrally dependent manner which operates using a polychromatic measurement beam bundle directed to the measurement object.
b) Description of the Related Art
Preferable measurement objects are surfaces reflecting in a spectrally dependent manner for radiation in the extreme ultraviolet range (EUV) which achieve reflectivity in a narrow spectral range because of their layer construction. Up to one hundred layer pairs of different materials, e.g., alternating layers of silicon and molybdenum, with layer thicknesses in the range of a few nanometers and accuracies of <0.1 nm RMS are to be provided.
The production process for optics of the type mentioned above, particularly for use in EUV lithography, requires effective on-site quality controls which must be carried out at high throughput. A wavelength-dependent determination of the maximum reflectivity and a determination of the bandwidth of the spectral reflection curve within close tolerances are required. Typically, accuracies of better than 1% are demanded.
Since the quality assurance must guarantee that the product is constructed homogeneously with respect to its reflection characteristics, test measurements must be carried out to check the constancy of the measured values in as many locations as possible.
For example, when a mask with dimensions of approximately 150×150 mm2 is to be measured with a spatial resolution of less than 0.2 mm2, a measurement time of one hour requires a measurement frequency of more than 33 Hertz, that is, a measurement time of less than 30 milliseconds for each individual measurement point of the approximately 120,000 measurement points.
As is well known, a spectral reflection curve is always recorded in accordance with measurement techniques and, through its evaluation, the relevant parameters or characteristic values are obtained. Reflection curves of the kind mentioned above are contained in FIGS. 1 and 2. The curve in FIG. 1 corresponds to an “ideal” multilayer coating for EUV radiation of 13.5 nm at an incident angle of 5°. Deviations in the curve will occur when there are defects in the coating, which leads in FIG. 2 to a shifting of the “central wavelength” and the maximum reflectivity. The illustrated change corresponds to a defect in the layer thickness of 1% and interdiffusion of 1 nm.
Of all the usual methods for recording the reflection curve, in which either the wavelength varies at constant incident angle (λ-scan) or the incident angle varies at constant wavelength (θ-scan), the λ-scan delivers the most meaningful results because the characteristic values can be read out directly.
It is known to determine the spectral reflection of a sample, which is given by the ratio of reflected to radiated spectral photon flow IR and IO, by means of reflectometers.
In the monochromatic concept, which can also be applied to synchrotrons, the radiation emitted by a polychromatic radiation source is directed in a monochromatic manner onto a measurement object and the reflected amplitude measured by means of an individual detector is compared with a separately emitted reference amplitude. (Windt, D. L. et al., “XUV Characterization Comparison of Mo/Si Multilayer Coating”, X-Ray/EUV Optics for Astronomy, Microscopy, Polarimetry, and Projection Lithography, Eds. R. B. Hoover and A. B. C. Walker, Jr., Proceedings of SPIE 1343 (1991), p. 274, and Gullikson, E. M., et al., “A soft X-ray/EUV reflectometer based on a laser produced plasma source”, Journal of X-Ray Science and Technology, October 1992, Vol. 3; (No. 4), 283-299).
The process to be repeated for all necessary wavelengths requires up to one hundred spectral measurement points per selected location on the measurement object and imposes high performance demands on the radiation source to achieve a sufficient photon flow per spectral measurement interval. Typical measurement times are in the range of one minute per location, for which reason this concept does not offer an effective quality control for series manufacture.
A polychromatic approach which is better suited to laboratory radiation sources provides for broadband irradiation of the measurement object and dispersion of the reflected radiation with a spectrograph prior to recording the different wavelengths by a line receiver or surface receiver (G. Schriever, et al., J. Appl. Optics, Vol. 37, No. 7, p. 1243 (1998). This is disadvantageous owing to the time required for the readout of the line receiver or surface receiver.
The recording of the spectral reflection curve which is always carried out in the art regardless of the concept applied disadvantageously increases expenditure on apparatus and impairs throughput by time-consuming measurements. The latter disadvantage occurs especially when the measurements are to be carried out on site, as is desirable. Since, in contrast to a synchrotron, the compact laboratory radiation sources or portable radiation sources required for this purpose have considerable disadvantages with respect to beam quality, particularly spectral brilliance, every attempt at complete characterization of a larger measurement object is hampered.